1. Field of the Invention
The present invention relates to an engine including a catalyst that purifies exhaust and to an outboard motor powered by the engine.
2. Description of the Related Art
With a vessel, even if engine overheating occurs, the engine is not stopped and the operation of the engine is continued in a state of restricted engine speed to return the vessel to port. With the outboard motor disclosed in Japanese Unexamined Patent Publication No. H11-72040, the engine speed is decreased by misfiring when the engine overheats. With the outboard motor described in U.S. Patent Application Publication No. 2002/0088429 A1, the engine speed is decreased by at least one of misfiring and stopping the fuel supply when the engine overheats.
When the engine output is decreased to decrease the engine speed, the amount of heat generated by the engine decreases and, therefore, the temperature rise rate of the engine decreases gradually and the temperature of the engine begins to decrease. However, as shall be described below, if the amount of heat generated by the engine is large, the temperature of the engine continues to rise even when the engine speed is decreased.
For example, with an engine that includes a catalyst, not only combustion heat but heat due to reaction of the catalyst and exhaust is also generated. Further with the engine that includes the catalyst, an air-fuel ratio (A/F) of an air-fuel mixture is set at a stoichiometric air-fuel ratio (air-fuel ratio at which the oxygen and the fuel in the air-fuel mixture react in just proportions) in many operation conditions and, therefore, the temperature of the exhaust is high. There is thus a possibility that the temperature rise rate of the engine cannot be decreased rapidly by simply decreasing the engine speed. Further with the conventional outboard motors mentioned above, exhaust containing uncombusted fuel is discharged from the combustion chamber because the engine speed is decreased by misfiring. Therefore, if a catalyst is installed in the engine, the exhaust containing the uncombusted fuel contacts the catalyst and accelerates the degradation of the catalyst.